Sale of Oil and Natural Gas Property Interests - Additional Information (Detail)	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($) a	Sep. 30, 2017 USD ($) a	Sep. 30, 2018 USD ($) a	Sep. 30, 2017 USD ($) a
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain (loss) on sale of assets	$ (6,000)	$ 13,000	$ 1,814,000	$ 12,000
Asset Sale 100 Acres [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain (loss) on sale of assets				0
Proceeds from sale of asset				$ 500,000
Area of land | a		100		100
Asset Sale 1,000 Acres [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain (loss) on sale of assets			1,500,000
Proceeds from sale of asset			$ 6,000,000.0
Area of land | a	1,000		1,000
Asset Sale 400 Acres [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain (loss) on sale of assets			$ 0
Proceeds from sale of asset			$ 3,800,000
Area of land | a	400		400
Asset Sale 50 Acres [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain (loss) on sale of assets			$ 300,000
Proceeds from sale of asset			$ 300,000
Area of land | a	50		50